Vegetation—Nourishment and Growth of Plants—Effects of the different
Rays of the Solar Spectrum—Classes—Botanical Districts.
IN the present state of the globe, a third part only of its surface is
occupied by land, and probably not more than a fourth part of that is
inhabited by man, but animals and vegetables have a wider range. The
greater part of the land is clothed with vegetation and inhabited by
quadrupeds, the air is peopled with birds and insects, and the sea teems
with living creatures and plants. These organized beings are not
scattered promiscuously, but all classes of them have been originally
placed in regions suited to their respective wants. Many animals and
plants are indigenous only in determinate spots, while a thousand others
might have supported them as well, and to many of which they have been
transported by man.
Plants extract inorganic substances from the ground, which are
indispensable to bring them to maturity, but the atmosphere supplies the
vegetable creation with the principal part of its food.
The black or brown mould which is so abundant is the produce of decayed
vegetables. When the autumnal leaves, the spoil of the summer, fall to
the ground, and their vitality is gone, they enter into combination with
the oxygen of the atmosphere, and convert it into an equal volume of
carbonic acid gas, which, consequently, exists abundantly in every good
soil, and is the most important part of the food of vegetables. This
process is slow, and stops as soon as the air in the soil is exhausted;
but the plough, by loosening the earth, and permitting the atmosphere to
enter more freely and penetrate deeper into the ground, accelerates the
decomposition of the vegetable matter, and consequently the formation of
carbonic acid.
In loosening and refining the mould, the common earth-worm is the
fellow-labourer with man; it eats earth, and, after extracting the
nutritious part, ejects the refuse, which is the finest soil, and may be
seen lying in heaps at the mouth of its burrow. So instrumental is this
creature in preparing the ground, that it is said that there is not a
particle of the finer vegetable mould that has not passed through the
intestines of a worm: thus, the most feeble of living things is employed
by Providence to accomplish the most important ends.
The food of the vegetable creation consists of carbon, hydrogen,
nitrogen, and oxygen—all of which plants obtain entirely from the
atmosphere in the form of carbonic acid gas, water, and ammonia. They
imbibe these three substances, and, after having decomposed them, they
give the oxygen to the air, and consolidate the carbon, water, and
nitrogen into wood, leaves, flowers, and fruit.
The vitality of plants is a chemical process entirely due to the sun’s
light; it is most active in clear sunshine, feeble in the shade, and
nearly suspended in the night, when plants, like animals, have their
rest.
The atmosphere contains only one two-thousandth part of carbonic acid
gas, yet that small quantity yields enough of carbon to form the solid
mass of all the magnificent forests and herbs that clothe the face of
the earth, and the supply of that necessary ingredient in the
composition of the atmosphere is maintained by the breath of animals, by
volcanos, and by combustion. The green parts of plants constantly imbibe
carbonic acid in the day; they decompose it, assimilate the carbon, and
return the oxygen pure to the atmosphere. As the chemical action is
feeble in the shade and in gloomy weather, only a part of the carbonic
acid is decomposed, then both oxygen and carbonic acid are given out by
the leaves; but during the darkness of the night a chemical action of a
different character takes place, and almost all the carbonic acid is
returned unchanged to the atmosphere, together with the moisture which
is evaporated from the leaves both night and day. Thus, plants give out
pure oxygen during the day, and carbonic acid and water during the
night.
Since the vivifying action of the sun brings about all these changes, a
superabundance of oxygen is exhaled by the tropical vegetation in a
clear unclouded sky, where the sun’s rays are most energetic, and
atmospheric moisture most abundant. In the middle and higher latitudes,
on the contrary, under a more feeble sun and a gloomy sky, subject to
rain, snow, and frequent atmospheric changes, carbonic acid is given out
in greater quantity by the less vigorous vegetation. But here, as with
regard to heat and moisture, equilibrium is restored by the winds; the
tropical currents carry the excess of oxygen along the upper strata of
the atmosphere to higher latitudes, to give breath and heat to men and
animals; while the polar currents, rushing along the ground, convey the
surplus carbonic acid to feed the tropical forests and jungles. Harmony
exists between the animal and vegetable creations; animals consume the
oxygen of the atmosphere, which is restored by the exhalation of plants,
while plants consume the carbonic acid exhaled by men and animals; the
existence of each is thus due to their reciprocal dependence. Few of the
great cosmical phenomena have only one end to fulfil, they are the
ministers of the manifold designs of Providence.
When a seed is thrown into the ground, the vital principal is developed
by heat and moisture, and part of the substance of the seed is formed
into roots, which suck up water mixed with carbonic acid from the soil,
decompose it, and consolidate the carbon. In this stage of their growth,
plants derive their whole sustenance from the ground. As soon, however,
as the sugar and mucilage of the seed appear above the ground, in the
form of leaves or shoots, they absorb and decompose the carbonic acid of
the atmosphere, retain the carbon for their food, give out the oxygen in
the day, and pure carbonic acid in the night. In proportion as plants
grow, they derive more of their food from the air and less from the
soil, till their fruit is ripened, and then the whole of their
nourishment is derived from the atmosphere. Trees are fed from the air
after their fruit is ripe till their leaves fall; annuals till they die.
Air-plants, and several species of cactus and others, derive all their
food from the atmosphere. It is wonderful that so small a quantity of
carbonic acid as exists in the air should suffice to supply the whole
vegetation of the world—and still more wonderful that a seed, minute
enough to be wafted invisibly by a breath of air, should be the theatre
of all the chemical changes that make it germinate.[153]
Plants absorb water from the ground by their roots; they decompose it,
and the hydrogen combines in different proportions with their carbonic
acid to form wood, sugar, starch, gum, vegetable, oils, and acids. As
the green parts combine with the oxygen of the air, especially during
night, when the functions of plants are torpid, it is assimilated on the
return of daylight, and assists in forming oils, resins, and acids. The
combination of the oxygen of the air with the leaves, and also with the
blossom and fruit, during night, is quite unconnected with the vital
process, as it is the same in dead plants. An acid exists in the juice
of every plant, generally in combination with an alkali. It must be
observed, however, that these different substances are produced at
different stages in the growth; for example, starch is formed in the
roots, wood, stalk, and seed, but it is converted into sugar as the
fruit ripens, and the more starch the sweeter the fruit becomes. Most of
these new compounds are formed between the flowering of the plant and
the ripening of the fruit, and indeed they furnish the materials for the
flowers, fruit, and seed.
Ammonia, the third organic constituent of plants, is the last residue
from the decay and putrefaction of animal matter. It is volatilized, and
rises into the atmosphere, where it exists as a gas, but in so small a
quantity that it is with difficulty detected by chemical analysis; yet,
as it is very soluble in water, enough is brought to the ground by rain
to supply the vegetable world. Ammonia enters plants by their roots
along with rain-water, and is resolved within them into its constituent
elements, hydrogen and nitrogen. The hydrogen aids in forming the wood,
acids, and other substances before mentioned; while the nitrogen enters
into every part of the plant and forms new compounds; it exists in the
blossom and fruit before it is ripe, and in the wood, as albumen; it
also forms gluten, which is the nutritious part of wheat, barley, oats,
and all other cerealia, as well as of esculent roots, as potatoes,
beet-root, &c. Nitrogen exists abundantly in peas, beans, and pulse of
every kind; it enters into the composition of most elementary vegetable
substances; in short, a plant may grow without ammonia, but it cannot
produce seed or fruit; the use of animal manure is to supply plants with
this essential article of their food. Thus, the decomposition and
consolidation of the elementary food of plants, the formation of the
green parts, the exhalation of moisture by their leaves, its absorption
by their roots, and all the other circumstances of vegetable life, are
owing to the illuminating power of the sun. Heat can be supplied
artificially in our northern climates, but it is impossible to replace
the splendour of a southern sun. His illuminating influence is displayed
in a remarkable degree by the cacalia ficoides; its leaves combine with
the oxygen of the atmosphere during the night, and are as sour as sorrel
in the morning; as the sun rises they gradually lose their oxygen, and
are tasteless at noon; by the continued action of light they lose more
and more, till towards evening they become bitter. The difference of a
clear or cloudy sky has an immense effect on vegetation; the ripening of
fruit depends upon the habitual serenity of the sky more than on summer
temperature alone.
The blue rays of the solar spectrum have most effect on the germination
of seed; the yellow rays, which are the most luminous, on the growing
plant. That is on account of the chemical rays, now so well known by
their action in Daguerreotype impressions. They are most abundant beyond
the visible part of the solar spectrum, and diminish through the violet,
blue, and green, to the yellow, where they cease. They penetrate the
ground, and have a much greater influence on the germination of seeds
than ordinary light or darkness. That invisible principle, together with
light, is essential to the formation of the colouring matter of leaves;
it is most active in spring, and is in very considerable excess compared
with the quantity of light and heat; but as summer advances the reverse
takes place; the calorific radiation, or those hot rays corresponding to
the extreme red of the spectrum, which facilitate the flowering and
forming of the fruit, become by far the most abundant; and a set of
invisible rays, which exist near the point of maximum heat in the solar
spectrum, are also most abundant in summer. Mr. Hunt found that the hot
rays immediately beyond the visible red destroy the colour of
palm-leaves; and for that reason the glass of the palm-house at Kew
Gardens is tinged pale yellow-green by oxide of copper, which excludes
the scorching rays in question, though it is permeable by the other rays
of heat, those of light, and the chemical rays.[154]
In spring and summer the oxygen taken in by the green leaves in the
night aids in the formation of oils, acids, and the other parts that
contain it; but as soon as autumn comes, the vitality or chemical action
of vegetables is weakened; and the oxygen, no longer given out in the
day, though still taken in during the night, becomes a minister of
destruction; it changes the colour of the leaves, and consumes them when
they fall. Nitrogen, so essential during the life of plants, also
resumes its chemical character when they die, and by its escape hastens
their decay.
Although the food which constitutes the mass of plants is derived
principally from water and the gases of the atmosphere, fixed substances
are also requisite for their growth and perfection, and these they
obtain from the earth by their roots. The inorganic matters are the
alkalis, phosphates, silica, sulphur, iron, and others.
It has already been mentioned that vegetable acids are found in the
juices of all the families of plants. They generally are in combination
with one or other of the alkaline substances, as lime, soda, potash, and
magnesia, which are as essential to the existence of plants as the
carbonic acid by which these acids are formed: for example, vines have
potash; plants used as dyes never give vivid colours without it; all
leguminous plants require it, and only grow naturally on ground that
contains it. None of the corn tribe can produce perfect seeds unless
they have both potash and phosphate of magnesia; nor can they or any of
the grasses thrive without silica, which gives the hard coating to
straw, to the beard of wheat and barley, to grass, canes, and bamboos;
it is even found in solid lumps in the hollows and joints of cane, known
in India by the name of tabashir. To bring the cerealia to perfection,
it is indispensable that in their growth they should be supplied with
carbonic acid for the plant, silica to give it strength and firmness,
and nitrogen for the grain.
Phosphoric acid, combined with an earth or alkali, is found in the ashes
of all vegetables, and is essential to many. Pulse contain but little of
it, and on that account are less nutritious than the cerealia. The
family of the cruciferæ, as cabbages, turnips, mustard, &c., contain
sulphur in addition to the substances common to the growth of all
plants; each particular tribe has its own peculiarities, and requires a
combination suited to it. On that account there is often a marked
difference in the arborescent vegetation on the same mountain, depending
on the nature of the rocks.
The ocean furnishes some of the matters found in plants; the prodigious
quantity of sea-water constantly evaporated carries with it salt in a
volatilized state, which, dispersed over the land by the wind, supplies
the ground with salt and the other ingredients of sea-water. The
inorganic matters which enter plants by their roots are carried by the
sap to every part of the vegetable system. The roots imbibe all liquids
presented to them indiscriminately, but they retain only the substances
they require at the various stages of their growth, and throw out such
parts as are useless, together with the effete or dead matter remaining
after the nutriment has been extracted from it. Plants, like animals,
may be poisoned, but the power they have of expelling deleterious
substances by their roots generally restores them to health. The
feculent matter injures the soil; besides, after a time the ground is
drained of the inorganic matter requisite for any one kind of plant:
hence the necessity for a change or rotation of crops.
A quantity of heat is set free and also becomes latent in the various
transmutations that take place in the interior of plants; so that they,
like the animal creation, have a tendency to a temperature of their own,
independent of external circumstances.
The quantity of electricity requisite to resolve a grain weight of water
into its elementary oxygen and hydrogen is equal to the quantity of
atmospheric electricity which is active in a very powerful
thunder-storm; hence, some idea may be formed of the intense energy
exerted by the vegetable creation in the decomposition of the vast mass
of water and other matters necessary for its sustenance. But there must
be a compensation in the consolidation of the vegetable food, otherwise
a tremendous quantity would be in perpetual activity. It is said to be
given out from the points of their leaves, so, possibly, some part of
the atmospheric electricity may be ascribed to this cause; but there is
reason to believe that electricity, excited by the power of solar light,
constitutes the chemical vitality of vegetation.
The colouring matter of flowers is various, if we may judge from the
effect which the solar spectrum has upon their expressed juices. The
colour is very brilliant on the tops of mountains and in the Arctic
lands. Possibly the diminished weight of the air may have some effect,
for it can scarcely be supposed that barometrical changes should be
entirely without influence on vegetation.
The perfume of flowers and leaves is owing to a volatile oil, which is
often carried by the air to a great distance: in hot climates it is most
powerful in the morning and evening. The odour of the Humiria has been
perceived at the distance of three miles from the coast of South
America, a species of Tetracera sends its perfume as far from the island
of Cuba, and the aroma of the Spice Islands is wafted out to sea. The
variety of perfumes is infinite, and shows the innumerable combinations
of which a few simple substances are capable, and the extreme minuteness
of the particles of matter.
In northern and mean latitudes, winter is a time of complete rest to the
vegetable world, and in tropical climates the vigour of vegetation is
suspended during the dry, hot season, to be resumed at the return of the
periodical rains. The periodical phenomena of the appearance of the
first leaves, the flowering, ripening of the fruit, and the fall of the
leaf, depend upon the annual and diurnal changes of temperature,
moisture, electricity, and perhaps on magnetism, and succeed with such
perfect harmony and regularity, that, were there a sufficient number of
observations, lines might be drawn on a globe passing through all places
where the leaves of certain plants appear simultaneously, and also for
the other principal phases of vegetation. In places where the same plant
flowers on the same day, the fruit may not ripen at the same period in
both; it would therefore be interesting to know what relation lines
passing through those would have to one another and to the isothermal
lines; more especially with regard to the plants indispensable to man,
since the periodicity of vegetation affects his whole social
condition.[155]
Almost all plants sleep during the night; some show it in their leaves,
others in their blossom. The Mimosa tribe not only close their leaves at
night, but their foot-stalks droop; in a clover-field not a leaf opens
until after sunrise. The common daisy is a familiar instance of a
sleeping flower; it shuts up its blossom in the evening, and opens its
white and crimson-tipped star, the “day’s eye,” to meet the early beams
of the morning sun; and then also “winking mary-buds begin to ope their
golden eyes.” The crocus, tulip, convolvulus, and many others, close
their blossoms at different hours towards evening, some to open them
again, others never. The ivy-leaved lettuce opens at eight in the
morning, and closes for ever at four in the afternoon. Some plants seem
to be wide awake all night, and to give out their perfume then only, or
at nightfall. Many of the jessamines are most fragrant during the
twilight: the Olea fragrans, the Daphne odorata, and the night-stock
reserve their sweetness for the midnight hour, and the night-flowering
Cereus turns night into day. It begins to expand its magnificent
sweet-scented blossom in the twilight, it is full blown at midnight, and
closes, never to open again, with the dawn of day;—these are “the bats
and owls of the vegetable kingdom.”[156]
Many plants brought from warm to temperate climates have become
habituated to their new situation, and flourish as if they were natives
of the soil; such as have been accustomed to flower and rest at
particular seasons change their habits by degrees, and adapt themselves
to the seasons of the country that has adopted them. It is much more
difficult to transfer alpine plants to the plains. Whether from a change
of atmospheric pressure or mean temperature, all attempts to cultivate
them at a lower level generally fail: it is much easier to accustom a
plant of the plains to a higher situation.
Plants are propagated by seeds, offsets, cuttings, and buds; hence they,
but more especially trees, have myriads of seats of life, a congeries of
vital systems acting in concert, but independently of each other, every
one of which might become a new plant. In this respect the fir and pine
tribe are inferior to deciduous trees, which lose their leaves annually,
because they are not easily propagated except by seeds. It has been
remarked that all plants that are propagated by buds from a common
parent stock have the same duration of life; this has been noticed
particularly with regard to some species of apple-trees in England. It
appears that all the garden varieties of fruit, whether from buds,
layers, or cuttings, wear out after a time; and that seedlings have a
great tendency to revert to the original wild character of the plant.
A certain series of transitions takes place throughout the lives of
plants, each part being transformed and passing into another; a law that
was first observed by the illustrious poet Göthe. For example, the
embryo leaves pass into common leaves, these into bracteæ, the bracteæ
into sepals, the sepals into petals, which are transformed into stamens
and anthers, and these again pass into ovaries with their styles and
stigmas, that are to become the fruit and ultimately the seed of a new
plant.
Plants are naturally divided into three classes, differing materially in
organization:—The Cryptogamia, whose flowers and seeds are either too
minute to be easily visible, or are hidden in some part of the plant, as
in fungi, mosses, ferns, and lichens, which are of the least perfect
organization. Next to these are the monocotyledonous plants, as grasses
and palms, in which the foot-stalks of the old leaves form the outside
of the stem; plants of this class have but one-seed-lobe, which forms
one little leaf in their embryo state. Their flowers and fruit are
generally referable to some law in which the number 3 prevails, as, for
example, the petals and other parts are three in number. The
dicotyledonous plants form the third class, which is the most perfect in
its organization, and by much the most numerous, including the trees of
the forest and most of the flowering shrubs and herbs. They increase by
coatings from without, as trees, where the growth of each year forms a
concentric circle of wood round the pith or centre of the stem: the
seeds of these plants have two lobes, which in their embryo state appear
first in two little leaves above ground, like most of the European
species. The parts of the flowers and fruit of this class generally have
some relation to the number 5.
The three botanical classes are distributed in very different
proportions in different zones: monocotyledonous plants, such as grasses
and palms, are much more rare than the dicotyledonous class. Between the
tropics there are four of the latter to one of the grass or palm tribes,
in the temperate zones six to one, and in the polar regions only two to
one, because mosses and lichens are most abundant in the high latitudes,
where dicotyledonous plants are comparatively rare. In the temperate
zones one-sixth of the plants are annuals, omitting the cryptogamia; in
the torrid zone scarcely one plant in twenty is annual, and in the polar
regions only one in thirty. The number of ligneous vegetables increases
on approaching the equator, yet in North America there are 120 different
species of forest-trees, whereas in the same latitudes in Europe there
are only 34. The social plants, grasses, heaths, furze, broom, daisies,
&c., which cover large tracts, are rare between the tropics, except on
the mountains and table-lands and on the llanos of equatorial America.
Equinoctial America has a more extensive and richer vegetation than any
other part of the world; Europe has not above half the number of
indigenous species of plants; Asia, with its islands, has somewhat less
than Europe; Australia, with its islands in the Pacific, still less; and
there are fewer vegetable productions in Africa than in any part of the
globe of the same extent.
Since the constitution of the atmosphere is very much the same
everywhere, vegetation depends principally on the sun’s light, moisture,
and the mean annual temperature, and it is also in some degree regulated
by the heat of summer in the temperate zones, and also by exposure, for
such plants as require warmth are found at a lower level on the north
than on the south side of a mountain. Between the tropics, wherever rain
does not fall, the soil is burnt up and is as unfruitful as that exposed
to the utmost rigour of frost; but where moisture is combined with heat
and light, the luxuriance of the vegetation is beyond description. The
abundance and violence of the periodical rains combine with the intense
light and heat to render the tropical forests and jungles almost
impervious from the rankness of the vegetation. This exuberance
gradually decreases with the distance from the equator; it also
diminishes progressively as the height above the level of the sea
increases, so that each height has a corresponding parallel of latitude
where the climates and floras are similar, till the perpetual snow on
the mountain-tops, and its counterpart in the polar regions, have a
vegetation that scarcely rises above the surface of the ground. Hence,
in ascending the Himalaya or Andes from the luxuriant plains of the
Ganges or Amazons, changes take place in the vegetation analogous to
what a traveller would meet with in a journey from the equator to the
poles. This law of decrease, though perfectly regular over a wide
extent, is perpetually interfered with by local climate and soil. From
the combination of various causes, as the distribution of land and
water, their different powers of absorption and radiation, together with
the form, texture, and clothing of the land, and the prevailing winds,
it is found that the isothermal lines, or imaginary lines drawn through
places on the surface of the globe which have the same mean annual
temperature, do not correspond with the parallels of latitude. Thus, in
North America the climate is much colder than in the corresponding
European latitudes. Quebec is in the latitude of Paris, and the country
is covered with deep snow four or five months in the year, and it has
occurred that a summer has passed there in which not more than 60 days
have been free from frost.
In the southern hemisphere, beyond the 34th parallel, the summers are
colder and the winters milder than in corresponding latitudes of the
northern hemisphere. Neither does the temperature of mountains vary
exactly with their height above the sea; other causes, as prevailing
winds, difference of radiation, and geological structure, concur in
producing irregularities which have a powerful effect on the vegetable
world.
However, no similarity of existing circumstances can account for whole
families of plants being confined to one particular country, or even to
a very limited district, which, as far as we can judge, might have grown
equally well on many others. Latitude, elevation, soil, and climate, are
but secondary causes in the distribution of the vegetable kingdom, and
are totally inadequate to explain why there are numerous distinct
botanical districts in the continents and islands, each of which has its
own vegetation, whose limits are most decided when they are separated by
the ocean, mountain-chains, sandy deserts, salt-plains, or internal
seas. Each of these districts is the focus of families and genera, some
of which are found nowhere else, and some are common to others, but,
with a very few remarkable exceptions, the species of plants in each are
entirely different or representative.[157] This does not depend upon the
difference in latitude, for the vegetation of the United States of North
America is totally unlike that of Europe under the same isothermal
lines, and even between the tropics the greatest dissimilarity often
prevails under different degrees of longitude: consequently, the cause
of this partial distribution of plants, and that of animals also, which
is according to the same law, must be looked for in those early
geological periods when the earth first began to be tenanted by the
present races of organized beings.
As the land rose at different periods above the ocean, each part, as it
emerged from the waves, had probably been clothed with vegetation, and
peopled with animals, suited to its position with regard to the equator,
and to the climate and condition of the globe then being. And as the
conditions and climate were different at each succeeding geological
epoch, so each portion of the land, as it rose, would be characterized
by its own vegetation and animals, and thus at last there would be many
centres of creation, as at this day, all differing more or less from one
another, and hence, alpine floras must be of older date than those in
the plains. The vegetation and faunas of those lands that differed most
in age and place would be most dissimilar, while the plants and animals
of such as were not far removed from one another in time and place would
have correlative forms or family likenesses, yet each would form a
distinct province. Thus, in opposite hemispheres, and everywhere at
great distances, but under like circumstances, the species are
representatives of one another, rarely identical: when, however, the
conditions which suit certain species are continuous, identical species
are found throughout, either by original creation or by migration. The
older forms may have been modified to a certain extent by the succeeding
conditions of the globe, but they never could have been changed, since
immutability of species is a primordial law of nature. Neither external
circumstances, time, nor human art, can change one species into another,
though each to a certain extent is capable of accommodating itself to a
change of external circumstances, so as to produce varieties even
transmissible to their offspring.
The flora of Cashmere and the higher parts of the Himalaya mountains is
similar to that of southern Europe, yet the species are representative,
not identical. In the plains of Tartary, where from their elevation the
degree of cold is not less than in the wastes of Siberia, the vegetation
of one might be mistaken for that of the other; the gooseberry, currant,
willow, rhubarb, and in some places the oak, hazel, cypress, poplar, and
birch, grow in both, but they are of different species. The flora near
the snow-line on the lofty mountains of Europe, and lower down, has also
a perfect family likeness to that in high northern latitudes. In like
manner many plants on the higher parts of the Chilian Andes are similar,
and even identical, with those in Tierra del Fuego; nay, the Arctic
flora has a certain resemblance to that of the Antarctic regions, and
even occasional identity of species. These remarkable coincidences may
be accounted for by the different places having been at an early
geological period at the same level above the ocean, and that they
continue to retain part of their original flora after their relative
positions have been changed. The tops of the Chilian Andes were probably
on a level with Tierra del Fuego when both were covered with the same
vegetation, and in the same manner the lofty plains of Tartary may have
acquired their vegetation when they were on the level of southern
Siberia.
In the many vicissitudes the surface of the globe has undergone,
continents formed at one period were broken up at another into islands
and detached masses by inroads of the sea and other causes. Now,
Professor E. Forbes has shown that some of the primary floras and faunas
have spread widely from their original centres over large portions of
the continents before the land was broken up into the form it now has,
and thus accounts for the similarity and sometimes identity of the
plants and animals of regions now separated by seas,—as, for example,
islands, which generally partake of the vegetation and fauna of the
continents adjacent to them. Taking for granted the original creation of
specific centres of plants and animals, Professor E. Forbes has clearly
proved that “the specific identity, to any extent, of the flora and
fauna of one area, with those of another, depends on both areas forming,
or having formed, part of the same specific centre, or on their having
derived their animal and vegetable population by transmission, through
migration, over continuous or closely contiguous land, aided, in the
case of alpine floras, by transportation on floating masses of ice.”
By the preceding laws the limited provinces and dispersion of animal and
vegetable life are explained, but the existence of single species in
regions very far apart has not yet been accounted for.
Very few of the exogenous or dicotyledonous plants are common to two or
more countries far apart: among the few, the Samolus Valerandi, a common
English plant, is a native of Australia; the Potentilla tridentata, not
found in Britain, except on one hill in Angusshire, is common to Arctic
Europe and the mountains of North America; and in the Falkland Islands
there are more than 30 flowering plants identical with those in Great
Britain.
There are many more instances of wide diffusion among the
monocotyledonous plants, especially grasses: the Phleum alpinum of
Switzerland grows without the smallest variation at the Straits of
Magellan, and Mr. Bunbury met with the European quaking-grass in the
interior of the country at the Cape of Good Hope; but the cellular or
cryptogamous class is most widely diffused—plants not susceptible of
cultivation, of little use to man, and of all others the most difficult
to transport. The Sticta aurata, found in Cornwall, is a native of the
Cape of Good Hope, St. Helena, the West Indian islands, and Brazil; the
Trichomanes brevisetum, long supposed to be peculiar to the British
isles, is ascertained to grow in Madeira, South America, &c.; and our
eminent botanist, Mr. Brown, found 38 British lichens and 28 British
mosses in New Holland, yet in no two parts of the world is the
vegetation more dissimilar; and almost all the lichens brought from the
southern hemisphere by Sir James Ross, amounting to 200 species, are
also inhabitants of the northern hemisphere, and mostly European.
In islands far from continents the number of plants is small, but of
these, a large proportion occur nowhere else. In St. Helena, of 30
flower-bearing plants, 1 or 2 only are native elsewhere, but in 60
species of cryptogamous plants Dr. Hooker found only 12 peculiar to the
island.
Some plants are more particularly confined to certain regions: the
species of Cinchona which furnish the Peruvian bark grow along the
eastern declivity of the Andes, as far as 18° S. lat.; the cedar of
Lebanon is indigenous on that celebrated mountain only; and the Disa
grandiflora is limited to a very small spot on the top of the
Table-mountain at the Cape of Good Hope; but whether these are remnants
whose kindred have perished by a change of physical circumstances, or
centres only beginning to spread, it is impossible to say.
Plants are dispersed by currents: of 600 plants from the vicinity of the
river Zaire on the coast of Africa, 13 are found also on the shores of
Guiana and Brazil, evidently carried by the great equatorial current to
countries congenial in soil and climate. The seeds of the Mimosa
scandens, the Guilandina Bonduc, and the cashew-nut, are wafted from the
West India islands to the coasts of Scotland and Ireland by the
Gulf-stream, a climate and soil which do not suit them, therefore they
do not grow. Of all the great orders, the species of Leguminosæ are most
widely dispersed on coasts, because their seeds are not injured by the
water. Winds also waft seeds to great distances; birds and quadrupeds,
and above all man, are active agents in dispersing plants.